Hybridization of plants is recognized as an important process for producing offspring having a combination of the desirable traits of the parent plants. The resulting hybrid offspring often has the ability to outperform the parents in different traits, such as in yield, adaptability to environmental changes, and disease resistance. This ability is called "heterosis" or "hybrid vigor". As a result, hybridization bas been used extensively for improving major crops, such as corn, sugar beet and sunflower. For a number of reasons, primarily related to the fact that most plants are capable of undergoing both self-pollination and cross-pollination, the controlled cross-pollination of plants without significant self-pollination, to produce a harvest of hybrid seeds, has been difficult to achieve on a commercial scale.
In nature, the vast majority of crop plants produce male and female reproductive organs on the same plant, usually in close proximity to one another in the same flower. This favors self-pollination. Some plants, however, are exceptions as a result of the particular morphology of their reproductive organs which favors cross-pollination. These plants produce hybrid offspring with improved vigor and adaptability. One such morphology in Cannabis ssp. (hemp) involves male and female reproductive organs on separate plants. Another such morphology in Zea mays (corn) involves male and female reproductive organs on different parts of the same plant. Another such morphology in Elaeis guineensis (oil palm) involves male and fertile female gametes which become fertile at different times in the plant's development.
Some other plant species, such as Ananas comosus (pineapple), favor cross-pollination through the particular physiology of their reproductive organs. Such plants have developed a so-called "self-incompatibility system" whereby the pollen of one plant is not able to fertilize the female gamete of the same plant or of another plant with the same genotype.
Some other plant species favor cross-pollination by naturally displaying the so-called genomic characteristic of "male sterility". By this characteristic, the plants' anthers degenerate before pollen, produced by the anthers, reaches maturity. See: "Male-Sterility in Higher Plants", M. L. B. Kaul, 1987, in: Monographs on Theoretical and Applied Genetics 10, Edit. Springer Verlag. Such a natural male-sterility characteristic is believed to result from a wide range of natural mutations, most often involving recessive deficiencies, and this characteristic can not easily be maintained in plant species that predominantly self-pollinate, since under natural conditions, no seeds will be produced.
Some other plants favor cross-pollination by natually displaying the character of "female-sterility" due to a deficient functioning of either the female gametophyte, the female gamete, the female zygote, or the seed. These plants produce no viable seeds. There are many different mutations that can lead to this condition, involving all stages of development of a specific tissue of the female reproductive organ. This characteristic distinguishes female-sterility from the more widely known phenomena of male-sterility and self-incompatibility. Although reducing the number of offspring a species can produce, the female-sterility trait has some evolutionary advantages in nature for some plants, especially for perennials. In perennials, the rate of vegetative growth is to a large extend determined by the distribution of biomass between vegetative and reproductive plant tissues. Female-sterile plants therefore tend to grow more vigorously than the female-fertile plants.
Although female-sterility inducing mutations probably occur as frequently as male-sterility inducing mutations, female-sterility inducing mutations are much less used in plant breeding and seed production and consequently much less studied, and only few examples of such mutations exist.
A well documented illustration of natural female-sterility is the oil palm (Elaeis guineensis) where the so-called "pisifera" condition is characterized by the inability of the developing seed to produce a shell. As a result, the developing seed aborts in an early stage, and no ripe fruit is formed. The gene encoding the "pisifera" genotype acts as a semi-dominant allele. Plants homozygous for the allele are not capable of producing a seed shell and consequently no ripe fruit or seeds. Plants heterozygotic for the allele produce ripe fruit and seeds with a thin shell (0.5 to 2 mm), while wild-type plants (which do not carry the allele) produce ripe fruit and seeds with shells of 2 to 6 mm thickness. These two genotypes are indistinguishable in seed yield, and their genotype is determined by that of the female parent plant. In oil palm breeding, the "pisifera" type is used as the male parent plant in all commercial seed production. By crossing pollen from the "pisifera" palms with the wild-type female parent plants, a homogeneous F1 hybrid seed population, producing thin-shelled fruit, is obtained.
Another example of a plant with a natural female-sterility, used for the commercial production of hybrid seed; is alfalfa. Alfalfa was known to have male-sterility genes, but in testing a hybrid seed production system in which male-sterile and male fertile plants were sown in separate bands, it appeared that a negligeable amount of hybrid seeds was produced. This low production was due to the fact that honeybees, responsible for pollination, have low affinity for male-sterile plants, favoring the self-pollination of the male-fertile plants. To obtain good seed set, it seemed necessary to interplant very closely to each other (thus not in separate rows) the male-fertile and the male-sterile parent plants. This was made possible when a female-sterility gene was discovered and bred into the male-fertile plants. Consequently, the only seeds which could be produced in the randomly sown plots were hybrid seeds obtained by cross-pollination between the female-sterile and the male-sterile parent plants.
For other crops, female-sterility has been reported, such as sorghum (Casady et al (1960) J. Hered. 51, 35-38), cotton (Tustus and Meyer (1963) J. Hered. 54, 167-168), tomato (Honma and Pratak (1964) J. Hered. 55, 143-145), wheat (Gotzov and Dzelepov (1974) Gen. Plant Breed. 7, 480-487), and pearl millet (Hanna and Powel (1974) J. Hered. 65, 247-249). There are, however, several problems in maintaining the female-sterile lines, and for this reason, such lines are not used on a commercially important scale.
Compared with male-sterility, the use of female-sterility offers some other advantages in the production of hybrid seeds. Female-sterility allows the production of fruits without seeds and enhanced vegetative biomass production and can, in some cases, induce more flower-setting within one season.